I think the Russian Sym hack can be adapted for other scripts also, and/or that they plan to work on these. But I'd check on the Kindle Developer's Corner forum. [update: yes, you can 'roll your own' Sym tables with the hack]

But using the Sym table is really awkward. At least for Latin-based scripts, it would be nice if they offered a keyboard input method that allowed you to cycle through diacritics of a given base character by holding the letter down to begin cycling through, then releasing the key when the variant you want appears. That could probably even be adopted for scripts like Cyrillic by mapping these to the nearest equivalent Latin letter.